1. Field of the Invention
The present invention relates to an image forming apparatus. More particularly, the present invention relates to an image forming apparatus that is capable of accurately distinguishing color photographic originals at least from color printed originals or black-and-white originals and which forms image according to the so distinguished type of original. The present invention also relates to an image forming apparatus capable of achieving faithful color density reproduction even from low-density, in particular, low-density low-contrast originals.
2. Prior Art
Image forming apparatus including various copiers capable of duplicating color originals, as well as color image printers are gaining increasing popularity these days.
In order to produce satisfactory color image with these color image forming apparatus, good balance must be attained not only in colors but also in densities.
With most of these image forming apparatus, in particular, color copiers, photographs and printed matter are used as color originals. However, different colorants are used in photographs and printed matter. Further, they have different spectral luminous efficiencies and require the use of copying materials having different spectral sensitivities. On account of these differences, the images of copies from photographic originals have had a different color balance than those from printed originals if they are duplicated under the same copying conditions. For example, color printed originals have a great overlap between the spectral density distributions of magenta and cyan inks. Thus, color printed originals have a high magenta density and color copiers that are adjusted to produce good copies of color printed originals are to be operated under copying conditions that provide suppressed magenta density. Therefore, if color photographic originals are duplicated under such image forming conditions, the production of magenta color is so limited as to form color copied images of green shades.
Color printed originals and color photographic originals also differ in the gradation of their image, so that if color photographic originals are duplicated on contrasty light-sensitive materials (i.e., those which are optimal for duplicating color printed originals) with copiers that employ silver halide photographic materials and various other light-sensitive materials, medium tone will not be effectively reproduced and only hard images will result. Therefore, in order to obtain an image of good quality with a single unit of image forming apparatus according to various types of color originals, it is necessary to identify the type of original to be duplicated and to select the proper image forming conditions (e.g. the amount by which color filters are adjusted and the amount of exposure) and the right kind of light-sensitive material according to the type of said original. However, this problem has not been fully considered in the design of prior art image forming apparatuses and if image forming conditions and light-sensitive materials are not changed in a single unit of image forming apparatus according to the type of original, it is impossible to obtain satisfactory image from either color printed originals or color photographic originals.
An image forming apparatus has also been proposed that enables the operator to manipulate selection keys according to such criteria as the presence or absence of halftone dots, thereby selecting appropriate image forming conditions that are optimal for the particular type of color original. However, the recent advances in color printing technology are so great that it is considerably difficult for unskilled operators to make accurate identification as to whether the original of interest is a color picture of the color printed matter obtained from color pictures. Further, the image forming process by operators is far from being efficient.
There have also been proposed several methods for identifying various types of color originals by means of image readers, as well as image forming apparatus that have such identifying means. However, in most identifying methods proposed so far, a plurality of functions involving measured values obtained from the image reader must be used in combination in order to identify the specific type of a color original of interest. In addition to this problem of complexity and difficulty in control, a further improvement is required of the precision that can be attained in the results of identification.
Prior art image forming apparatuses that are intended to duplicate color originals have various kinds of light-sensitive materials in stock that are optimal for the specific types of color originals to be duplicated but they are not usually furnished with light-sensitive materials dedicated to black-and-white originals and instead they form black-and-white image on color light-sensitive materials. Thus, standard exposure conditions that are optimal for black-and-white originals such as black-and-white photographs and printed matter have not been fully taken into account in the design of image forming apparatus of the type described above. As a result, if a black-and-white original is duplicated on a low-contrast, light-sensitive material that is optimal for duplicating color photographic originals (which is hereunder referred to as a "soft light-sensitive material"), the copied image will have a red shade as in the case where color printed originals are duplicated on soft light-sensitive materials and no satisfactory black-and-white copied image can be obtained. This is true whether the black-and-white original is a photograph or printed matter. It has therefore been desired to develop an apparatus that is capable of distinguishing color photographic originals not only from color printed originals but also from black-and-white originals.
In prior art image forming apparatus, the photometric region of the same length as, for example, the size of the light-sensitive material on which image is to be formed is often scanned either during prescanning or before reading the necessary information from the original of interest. All the data thus read are used to perform mathematical operations, thereby identifying the type of original to be duplicated. If, under these circumstances, the size of document to be copied is smaller than a predetermined area of the platen glass or the area of photometric region which is equal to the light-sensitive material of interest as in the case of copying color documents of small size, in particular, making enlarged copies of color prints of size E (82 mm.times.116 mm) or copying color documents of irregular sizes, or if the document is not properly placed on the platen glass, the document illuminating light that passes through the platen glass on a position way off the document during photometric scanning of the document surface is sometimes reflected by the white underside of the top cover to be directly launched into sensors. As a result, even the white area of the top cover is read as part of the image of document and subjected to mathematical operations. This can cause failure to correctly identify the document type and the color photographic original is detected either as a color printed original or as an intermediate original containing both color photographic and printed images.
A need sometimes arises to copy a bulky material such as a package or commercial goods with the top cover left open or to copy a certain page or pages of a thick book placed face down on the platen glass, again with the top cover left open. In these cases as well the case where the color document to be copied is smaller than the light-sensitive material on which image is to be formed, the following big problems will inevitably occur. If the document to be copied has a smaller image forming area than the photometric region or is a thick book that forms a large hollow portion around it or along the center margin which should inherently belong to the document region, the document illuminating light will pass unimpeded through the photometric region where no document is present or through the area of the platen glass corresponding to said hollow portion. If this occurs in the copying process, an area that hardly produces reflected light will occur in part of the document region which is inherently supposed to receive some reflected light. In other words, the occurrence of this phenomenon allows sensors to read such defective region as one of very high density and if mathematical operations for identification are performed on the basis of the resulting data, one often fails to identify the correct document type.
Some versions of the prior art image forming apparatus are capable of selecting an optimum light-sensitive material and setting optimum standard exposure conditions according to the document type identified by the conventional methods described above, and then performing exposure under said standard conditions to form image. With such color image forming apparatus, in particular, color copiers, color image forming conditions are initially set at the time of system installation or thereafter adjusted periodically so as to insure the formation of reproduced image having colors and densities faithful to the image of color original. With silver halide color image forming apparatus, photographic documents which require faithful reproduction of medium tone are copied on soft (low contrast) light-sensitive materials having comparatively low gamma-characteristics whereas printed documents which also require good contrast are copied on normal (contrasty) light-sensitive materials having comparatively high gamma-characteristics. In addition, the color image forming conditions are varied in such a way as to optimize the densities and color reproduction that are required for ideal image reproduction.
Color reflection type originals generally have a gamut of various densities and colors, so the average density of either the whole document or the image areas of the document is detected from the image of document and correction is made in such a way as to provide satisfactory color reproduction for the detected average density. If, in the case where image is to be formed on a light-sensitive material having the characteristics shown by curve a in FIG. 15, the overall density (e.g. average density) Do of a color printed original is found to be equal to Dc, the overall color density Dp of a reproduced image is equal to Da. Since Da&gt;Dc, in order to provide satisfactory color density reproduction, the characteristic curve a of the light-sensitive material is laterally shifted until Dp becomes equal to Dc. In other words, density correction is effected in such a way that when the density of original image Do is Dc, the density of reproduced image Dp is equal to Db (=Dc).
On the other hand, most black-and-white copiers are so designed that background areas of light colors and the nearly white background are rendered white by eliminating the light colors through proper density correction typically exemplified by an increase in the quantity of light.
Most frequently used color reflection type originals are such that many of the image areas present have densities equal to the average density but even documents having an overall dark density sometimes contain characters of lighter density or areas of medium density. If such documents are read for density correction, not only the dark areas but also the areas of light and medium density are scanned simultaneously and the document of interest is judged to be a dark document. Accordingly, density control is effected in such a way as to reproduce a darker image on the dark document. A problem with this approach of control for density correction is that although colors of high density are reproduced satisfactorily, colors in the low-density areas are skipped.
Conversely, if documents that have a low average density but which exhibit high contrast between a wide area of the white background and characters are corrected to produce a darker density by reducing the amount of light, fogging occurs in the white background which hence does not become snow-white.
Normal contrasty light-sensitive materials usually have comparatively high gamma-values and their characteristic curve has a large gradient that departs from the straight line of .gamma.=1 in FIG. 15. Reproduction with such materials is poor in the low-density areas. To cope with this problem, documents having an overall low density may be controlled in such a way that the quantity of light is not reduced to effect compensation for color densities. However, with documents such as maps that have an overall low density but which have background areas of relatively high density and light colors, if density correction is performed in such a way as to eliminate the color of background areas by increasing the quantity of light (i.e., the type of correction method employed with black-and-white copiers) or if the amount of light is not sufficiently reduced to effect compensation for color densities, the background areas will not assume a full color and will instead remain white, with consequent formation of an image that poorly reproduces colors in the low-density areas. With contrasty normal light-sensitive materials, satisfactory color reproduction is possible if the original is printed matter but no satisfactory colors can be produced in the low-density areas.
Thus, prior art image forming apparatuses which are incapable of distinguishing low-density originals with a wide area of the white background from low-density, low-contrast originals such as maps having background areas of high relatively density and light colors have failed to automatically produce images with efficient reproduction of color densities from either type of originals.